Let's call the missing side ? as shown in the problem.
Now set up a equation where everything adds up to 28x+11
?+8x+2+8x+2+5x+12=28x+11
?+21x+16=28x+11
?=28x-21x+11-16
?=7x-5
So 7x-5 is the missing side!
Hope it helps :D
